Over the last few years, the need has arisen in the semiconductor industry for inspection of unpatterned wafers which have a relatively rough surface as compared to regular monitor wafers, which are highly polished, specularly reflective wafers. Whereas the latter are typically bare unprocessed wafers, the former have usually been subjected to one or several process steps, frequently deposition of either dielectric or metallic layers.
In principle such substrates may be inspected with the present bare wafer inspection instruments. However, it is generally found that the results are disappointing. For maximum sensitivity, these instruments are optimized to collect nearly all scattered light from the inspected surface. The surface roughness on these rough wafers gives rise to a large and often strongly fluctuating background, which tends to obscure the response of small particles.
In recent years, patterned wafer inspectors, such as the Tencor 7000, have been developed for inspecting very rough surfaces. Here the roughness is a result of the structured topography of the patterns themselves, rather than from microscopic surface or film roughness. The nature of the scatter on these surfaces dictates a set of conditions which is sometimes extreme, and therefore drastic measures must be taken in order to discriminate against or eliminate the background scatter. In this process the light from small particles may get reduced to a level which makes detection difficult if not impossible. In practice it is indeed found that these instruments cannot detect particles as small as can be detected by a bare wafer scanner (at best they can detect 0.4 to 0.5 micron particles on a relatively smooth surface versus 0.1 micron for bare wafer scanners). In addition, the presence of the periodic patterns requires electronics and algorithms, which significantly increases the complexity and hence the cost of the instrument. There exists therefore a need for an instrument which has better sensitivity and is simpler than patterned wafer detectors for use on unpatterned but rough wafers, and with sensitivity approaching the bare (smooth) wafer scanners, if possible.
Moreover during the last two years, interest has arisen in cluster-tool type processing equipment, which allows a sequence of processing steps and a degree of cleanliness which cannot be accommodated in classical processing equipment. Associated with this cluster type processing equipment is a new kind of cluster type measurement instrumentation.
Very often the requirements for this type of instrumentation are vastly different from the classical case. It is not uncommon that the measurement is required to take place in vacuum, which places severe restriction on the type of equipment which can be used. The measurement equipment is very often dedicated to the particular cluster, which necessitates a large number of instruments in any single fab. Hence, it is desirable to design instruments which are very compact, highly reliable and relatively inexpensive to fabricate and to maintain.
Preferably, the instrument should have multiple measurements capability. For example, in inspecting a deposited dielectric film, it may be desirable to measure not only the number of particles inadvertently deposited in or upon the film, but also its surface roughness, reflectance, film thickness and if applicable film stress. It is therefore very desirable that these instruments use a common platform, and use as many parts and functions in common as possible to perform these tasks.